Bibliographic details of the publications referred to by author in this specification are collected alphabetically at the end of the description. The disclosure of each reference referred to in this application is incorporated herein by reference.
Reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in any country.
Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by the presence of misfolded protein depositions or amyloid plaques Crouch, P. J., S.-M. E. Harding, et al. (2008). “Mechanisms of Aβ mediated neurodegeneration in Alzheimer's disease.” Int J Biochem Cell Biol 40(2): 181-198. Plaques consist predominantly of amyloid β-peptide (Aβ), which is produced by cleavage from the membrane-bound amyloid precursor protein (APP) via the β/γ secretase pathway. (The sequence of Aβ is set out in SEQ ID NO 1). However, the current view suggests that soluble Aβ oligomer intermediates, and not the plaque burden, may be the major drivers of Aβ-mediated neuronal dysfunction Walsh, D. M. and D. J. Selkoe (2007). “Aβ oligomers—a decade of discovery.” J. Neurochem. 101(5): 1172-1184. Shankar, G. M., S. Li, et al. (2008). “Amyloid-beta protein dimers isolated directly from Alzheimer's brains impair synaptic plasticity and memory.” Nat. Med. Frustratingly, obtaining atomic resolution information for Aβ and such oligomers has been a major challenge Kajava, A. V., J. M. Squire, et al. (2006). “β-structures in fibrous proteins.” Adv Protein Chem 73: 1-15. Nelson, R. and D. Eisenberg (2006). “Recent atomic models of amyloid fibril structure.” Curr Opin Struct Biol 16(2): 260-265. Nelson, R. and D. Eisenberg (2006). “Structural models of amyloid-like fibrils.” Adv Protein Chem 73: 235-282, due in part to the propensity of the peptide to form amyloidal fibrils and aggregates rather than form crystallographic lattices.
The present inventors have obtained a 2.2 Å resolution crystal structure of residues 18-41 of Aβ peptide constrained within the CDR3 loop region of a shark immunoglobulin new antigen receptor (IgNAR) single variable domain antibody Henderson, K. A., V. A. Streltsov, et al. (2007). “Structure of an IgNAR-AMA1 complex: targeting a conserved hydrophobic cleft broadens malarial strain recognition.” Structure 15(11): 1452-66. The predominant oligomeric species is a tightly associated Aβ dimer, with paired dimers forming a tetramer which is caged within four IgNAR domains, preventing further uncontrolled amyloid formation. The results reveal unusual Aβ loop topologies and inter-peptide interactions, strikingly different from fibrillar models based on solid state NMR spectroscopy data Petkova, A. T., Y. Ishii, et al. (2002). “A structural model for Alzheimer's b-amyloid fibrils based on experimental constraints from solid state NMR.” Proc Natl Acad Sci USA 99(26): 16742-16747. Luhrs, T., C. Ritter, et al. (2005). “3D structure of Alzheimer's amyloid-β(1-42) fibrils.” Proc Natl Acad Sci USA 102(48): 17342-17347. Petkova, A. T., W. M. Yau, et al. (2006). “Experimental constraints on quaternary structure in Alzheimer's β-amyloid fibrils.” Biochemistry 45(2): 498-512. Sato, T., P. Kienlen-Campard, et al. (2006). “Inhibitors of amyloid toxicity based on β-sheet packing of Aβ40 and Aβ42.” Biochemistry 45(17): 5503-5516, that describe Aβ residues 18-42 as forming parallel, in-register β-sheets. Notwithstanding, conserved elements can be identified within the structure consistent with residues and motifs previously identified as critical in Ap peptide folding and neurotoxicity. This crystallographic model suggests a novel paradigm for Aβ oligomer formation, and potentially provides a system for the testing of Aβ oligomer imaging reagents and Alzheimer's disease drug candidates.